Commercial production of nitric acid is based on the ninety year old Ostwald process, and can be broken down into three major process steps:
I: Ammonia Oxidation EQU 4 HN.sub.3 (g)+5 O.sub.2 (g).fwdarw.4 NO(g)+6 H.sub.2 O(g) (1)
II: Nitric Oxide Oxidation and Nitrogen Dioxide Dimerization EQU 2 NO(g)+O.sub.2 (g).fwdarw.2 NO.sub.2 (g) (2) EQU 2 NO.sub.2 (g).fwdarw.N.sub.2 O.sub.4 (g) (3)
and
III: Dinitrogen Tetroxide Absorption EQU 3 N.sub.2 O.sub.4 (g)+2 H.sub.2 O(l).fwdarw.4 HNO.sub.3 (aq)+2 NO(g)(4)
The overall stoichiometry for the above reactions is given by the following formula: EQU NH.sub.3 +2 O.sub.2 .fwdarw.HNO.sub.3 +H.sub.2 O (5)
These reactions are performed in four major process units: an ammonia converter, a cooler and condenser apparatus ("cooler/condenser apparatus"), an absorption tower, and a bleacher. These units, along with equipment used for tail gas treatment, form the major units operating in a modern nitric acid plant. Other equipment includes piping or other means through which raw materials (including air) are routed, including piping carrying liquid that contains nitric acid.
A simplified process flow diagram showing the process units of a conventional nitric acid plant is shown in FIG. 1. Ammonia oxidation occurs exclusively in the ammonia converter 5. Typically, this reaction is catalytic and occurs over a platinum containing wire gauze, although a small number of plants use a cobalt based pelletized catalyst. Air 1a and ammonia 2 are premixed and in some cases preheated before being routed to the converter 5. The concentration of ammonia in the converter feed is generally held to between 9 and 11%. The desired reaction taking place in the converter is reaction (1). Important competing reactions, however, are: EQU 2 NO(g).fwdarw.N.sub.2 (g)+O.sub.2 (g) (6) EQU and EQU 4 NH.sub.3 (g)+6 NO(g).fwdarw.5 N.sub.2 (g)+6 H.sub.2 O(g) (7)
Ammonia oxidation is performed commercially at pressures ranging from 1 to 10 atmospheres and temperatures ranging from 1500 to 1700.degree. F. The choice of operating pressures is based primarily on downstream considerations, with most plants operating at about 10 atmospheres. The operating temperature is chosen, in part, by balancing nitric oxide yield, which increases with increasing temperature, and catalyst loss which also increases with increasing temperature. Most converters are operated at a temperature near 1650.degree. F.
The molar yield of nitric oxide formed from ammonia is generally near 95%, but varies depending on the reaction conditions employed. In particular, while approximately 100% ammonia is converted to other products, the yield of nitric oxide is lower than 100% because of competing reactions (6) and (7). Temperature, converter configuration, flow velocity, and the ratio of oxygen to ammonia in gas entering the converter affect the yield of nitric oxide.
The nitric oxide formed in the converter 5 must be converted into dinitrogen tetroxide to produce nitric acid in the absorption tower 15. This is done by first allowing the nitric oxide to react with the excess oxygen between the ammonia converter 5 and the absorption tower 15 to form nitrogen dioxide. The nitrogen dioxide is then allowed to dimerize to form dinitrogen tetroxide. These reactions occur primarily within the cooler/condenser apparatus.
Process equipment other than the cooler/condenser apparatus is present between the ammonia converter 5 and the absorption tower 15. This equipment may include, but is not limited to, a waste heat boiler, an economizer, a platinum filter, and piping or other means to connect these to each other and/or the converter 5 and the absorption tower 15. The combination of this equipment and the cooler/condenser apparatus is referred to herein as the "cooler/condenser train" 10.
The cooler/condenser apparatus is essentially a heat exchanger and a phase separator. Removing heat in the cooler/condenser promotes both phase separation and the oxidation of nitric oxide (per equation (9) below). The effect of temperature is particularly important in the cooler/condenser because the oxidation of nitric acid is a surprisingly slow homogeneous gas phase reaction with a rate which slows with increasing temperature. The phase separator condenses and removes water formed in the converter. The condensed water generally contains up to 50 wt. % nitric acid as a result of absorption of dinitrogen tetroxide (per reaction (4)). This weak acid solution 12 is normally pumped to the middle of the absorption tower 15.
Some plants route additional air through line 1c, shown in phantom, to the cooler/condenser train 10 to increase the oxygen partial pressure in the cooler and condenser apparatus.
Because operating conditions in the cooler/condenser are not normally at equilibrium, the rate of nitrogen dioxide formation can be calculated according to the following formula (the reverse reaction of reaction (2) can generally be ignored): EQU dP.sub.NO2 /dt=k(P.sub.NO).sup.2 (P.sub.O2) (8)
where the rate constant, k, as a function of temperature (in degrees Kelvin) is given as: EQU k=10.sup.(641/T)-(0.725) (9)
From equation (8), it is evident that high oxygen partial pressures increase the rate of reaction (2), reducing the volume necessary to oxidize a given amount of nitric oxide in the cooler/condenser. Because of this, nitric acid plants are normally run at medium to high pressures (e.g., about 3-10 atm.). Some plants also route additional air to the cooler condenser to increase oxygen partial pressure.
The dinitrogen tetroxide rich process gas 11 from the cooler/condenser is then contacted with water 13 in an absorption tower 15. Typically, the absorption tower is strayed, although packed towers are sometimes used. Normally, water 13 enters at the top of the absorption tower, weak acid 12 produced in the cooler/condenser apparatus enters in the middle of the tower, and process gas 11 in combination with additional air (usually referred to as secondary air) enters the absorption tower 15 near its bottom via line 17. The secondary air is provided by line 1b and is routed first through the bleacher 20. Air may also be provided directly to the absorber 15 through air line 1d. The product acid 16 is withdrawn from the bottom and a NOx containing vent gas ("tail gas") 14 exits from the top.
In the absorption tower, the dinitrogen tetroxide is reactively absorbed in the water and forms nitric acid. This reactive absorption is generally represented as reaction (4). However, the actual mechanism is thought to be: EQU N.sub.2 O.sub.4 (g).fwdarw.N.sub.2 O.sub.4 (l) (10) EQU N.sub.2 O.sub.4 (l)+H.sub.2 O(l).fwdarw.HNO.sub.3 (l)+HNO.sub.2 (l)(11)
The nitrous acid (HNO.sub.2) produced in reaction (11) either decomposes or oxidizes to form nitric acid: EQU 3 HNO.sub.2 (l).fwdarw.HNO.sub.3 (l)+H.sub.2 O(l)+2 NO(g) (12) EQU 2 HNO.sub.2 (l)+O.sub.2 (l).fwdarw.2 HNO.sub.3 (l) (13)
Additional nitrous acid is also formed by the absorption of dinitrogen trioxide, which is in turn produced in small quantities from the reaction of nitric oxide and nitrogen oxide, according to the following reactions. EQU NO(g)+NO.sub.2 (g).fwdarw.N.sub.2 O.sub.3 (g) (14) EQU N.sub.2 O.sub.3 (g).fwdarw.N.sub.2 O.sub.3 (l) (15) EQU N.sub.2 O.sub.3 (l)+H.sub.2 O(l).fwdarw.2 HNO.sub.2 (l) (16)
Although usually a minor reaction, nitrogen dioxide can also be adsorbed according to the following reactions: EQU NO.sub.2 (g).fwdarw.NO.sub.2 (l) (17) EQU 2 NO.sub.2 (l)+H.sub.2 O(l).fwdarw.HNO.sub.3 (l)+HNO.sub.2 (l)(18)
If the nitric oxide formed in reaction (12) is not reoxidized through reaction (2), it passes out of the tower in the tail gas. Loss of nitric oxide in this fashion reduces nitric acid yield and leads to higher emissions of NOx from the plant. Because NOx emissions are normally highly regulated, it is advantageous to reoxidize as much nitric oxide as possible.
Reoxidation of some of the nitric oxide occurs in spaces between the absorption tower trays. In a conventional plant, secondary air via line 17, and sometimes via line 1d (shown in phantom) are added to the absorption tower to increase the rate of reoxidation. By increasing the oxygen partial pressure, the secondary air also promotes reaction (13).
The nitric acid 16 removed from the base of the absorption tower typically contains dissolved unreacted dinitrogen tetroxide, nitrous acid, dinitrogen trioxide, and nitrogen dioxide. These impurities discolor the nitric acid, imparting a yellow color. The yellow color is caused by dissolved dinitrogen tetroxide and nitrogen dioxide. Nitrous acid and dinitrogen trioxide impart a blue color (which mixes with the yellow to form a green solution). A nitric acid and water mixture that lacks these impurities is clear, and is referred to as "water white".
These impurities can interfere with normal uses of nitric acid, particularly chemical syntheses, and must be removed. Since the removal of the impurities reduces the yellow color, this process step is known as bleaching.
To remove the impurities, the nitric acid is contacted in a countercurrent manner with an ascending stream of air provided to the bleacher 20 via line 1b. The bleacher is a strayed or packed tower that is normally either a separate unit from the absorption tower, or is formed by the bottom few trays of the absorption tower. (A combination of bleacher trays within the absorption tower and a separate bleacher tower is sometimes employed.)
The ascending stream of air in the bleacher physically strips the dissolved gases from the acid and chemically oxidizes the impurities. The two oxidation reactions which remove impurities are reactions (13) and (2). Reaction (13) removes unwanted color in the product by removing nitrous acid. Reaction (2) also removes nitrous acid by removing nitric oxide, thereby shifting reaction (12) to the right. The nitric acid 21 removed from the bleacher 20 is usually ready to be sold or consumed on site.
The air and NOx (collectively 17) exiting the bleacher are routed back to the absorption tower 15. This increases the oxygen partial pressure in the absorption tower and allows the NOx components stripped in the bleacher to be absorbed and form nitric acid.
The tail gas 14 contains significant amounts of NOx and, to conform to conventional environmental regulations, must be treated before being vented to the atmosphere. Currently in the United States, emissions from newly constructed nitric acid plants must be limited to 1.5 kg NOx per metric ton of acid (100% basis), which is equivalent to 230 ppm. To achieve this level of emissions, three abatement methods are conventionally used: absorption, adsorption, and catalytic reduction.
In the absorption process for abating NOx, tail gases are passed through one or more absorbers containing water or a solution of ammonia, urea, or sodium hydroxide. Where water is used, a weak acid solution is formed in the absorption process, which is recycled. Using other absorbants, nitrogen oxides are normally recovered as a nitrite/nitrate solution, which is used for fertilizer production. By employing these methods, current nitric acid producing plants are able to reduce NOx emissions in tail gas to less than 200 ppm.
It is known that adding supplemental oxygen can boost nitric acid production while controlling NOx emissions. Such addition of oxygen is described, for example, in U.S. Pat. Nos. 4,183,906; 4,183,906; 4,235,858; and 5,167,935; UK Patent No. 803211; and EP published Patent application Nos. 799794 and 808797. It is also described in Kongshaug, Extension of Nitric Acid Plant Capacity by Use of Oxygen, Nitric Acid Symposium (1981); and by Faried et al., Boosting Existing Nitric Acid Production, The Fertiliser Society (1986).
FIG. 2 is a further flow diagram, showing prior art techniques for adding supplemental oxygen to conventional nitric acid processes as described in the aforesaid European patent application EP 808797. In particular, supplemental oxygen may be added through line 30e to the ammonia converter, lines 30a and 30c to the cooler/condenser 10, line 30d to the absorption tower 15, line 30c to the ammonia converter 5, and/or line 30b to the bleacher 20.
While the oxygen can be added at several possible locations in the process, it must eventually be routed to the absorption tower. This allows secondary air to be rerouted to the converter (because nitric oxide can be oxidized using significantly less secondary air). Rerouting secondary air to the converter causes the total air flow to the converter to be increased. Because it is standard practice to keep the ammonia-to-air ratio constant, the ammonia flow is increased a corresponding amount, causing an overall increase in the amount of nitric acid produced.
Thus, it is highly advantageous to reroute secondary air to the converter to increase nitric acid production. However, to comply with environmental regulations this normally must be done without a corresponding increase in NOx emissions.
Maintaining NOx emissions at acceptable levels can be accomplished by direct oxygen injection, increasing the height of the absorption tower, raising plant pressure, or increasing the capacity of the tail gas treatment unit. It is also possible to add a separate stripping reactor, as shown e.g., in U.S. Pat. No. 4,062,928. However, direct oxygen injection, i.e., the addition of oxygen from a source that is separate from the primary and secondary air source, is advantageous because it involves a significantly lower capital expense. It has been determined, for example, that one ton of directly injected oxygen results in an increase of about one ton of nitric acid production, while maintaining NOx emissions at about the same levels. The increase in nitric acid is normally accomplished by rerouting secondary air from the bleacher to the converter (where a corresponding amount of ammonia can be added, as explained above).
However, it has been determined by the present inventors that rerouting secondary air from the bleacher can adversely affect color quality of the nitric acid. This is believed to be caused by the fact that substituting direct oxygen injection for secondary air in the bleacher reduces the amount of physical stripping of impurities that occurs. In particular, the loss of nitrogen (which is normally found in the secondary air but not in directly injected oxygen) reduces the amount of physical stripping that occurs.
Also, it would be advantageous to reduce the amount of supplemental oxygen required for direct oxygen injection, while maintaining the elevated nitric acid production level brought about by direct oxygen injection, and avoiding an increase in NOx emissions.
Methods of mixing gases, including oxygen, and liquids are described in U.S. Pat. Nos. 5,108,662; 5,356,600; 4,867,918; 5,061,406; 5,302,325; and 4,931,225. These patents do not disclose any practical advantage of these methods in mixing of gases and liquids in nitric acid production.
It is therefore an object of the present invention to allow an increase in the production of nitric acid, or a decrease in the amount of secondary oxygen supplied to a process for nitric acid production, while maintaining the color quality of the nitric acid produced, while maintaining NOx emissions at about the same levels, and without significant capital expenditure.